At present, soft drinks such as juices, or alcoholic beverages such as beer are sold in a form that the drink is filled in containers such as glass bottles or PET (polyethylene terephthalate) bottles. When the drinks are on sale, in order to differentiate the products from other products and to improve its visibility to enhance its commercial value, a heat-shrinkable label on which printing is given is often provided onto the outside of each container.
Among the above heat-shrinkable label, for label use of PET bottles with potential increase of demand, a heat-shrinkable film is requested to be able to obtain excellent shrink finishing quality within relatively short time at low temperature and to have small natural shrinkage ratio. One of the reasons is that temperature of the labeling process of the heat-shrinkable film to be fitted with recent PET bottles is required to be colder. In other words, nowadays, it is common that steam-shrinker is used to make the heat-shrinkable film shrink and to label bottles. However, in order to avoid quality loss of the content due to the heat increase, it is desirable to carry out shrinking process of the film at as low temperature as possible. This is why recent heat-shrinkable film industry is developing a heat-shrinkable film which starts shrinking in the steam-shrinker in a time of labeling at as low temperature as possible and which is able to obtain excellent shrink finishing quality after passing through the steam-shrinker.
As materials of the heat-shrinkable film, for example, a polyethylene terephthalate series resin and a polystyrene series resin are used. An elongated film formed by the above resins has high transparency, brilliance, and stiffness, together with excellent shrinkage property in low-temperature; thereby such an elongated film can be suitably used as a heat-shrinkable film. On the other hand, a polyolefin (hereinafter, it may be abbreviated as “PO”.) series resin is a material having desirable features such that problems related to combustion product gas and endocrine-disrupting compound (which is the so-called “environmental hormone”) are relatively few, and such that density thereof is low, which contributes for decreasing weight of waste. Nevertheless, the heat-shrinkable film composed by this PO series resin has problems such that stiffness and shrinkage property in low temperature is not sufficient, shrinkage in a time of heat shrinkage is insufficient, further natural shrinkage (it is the circumstance where a film shrinks before its original use at slightly higher temperature than room temperature, e.g. in summer season.) is easily caused.
Moreover, the above polyethylene terephthalate series resin, polystyrene series resin, and PO series resin are petroleum-derived resin; therefore there is a problem of exhaustion of crude oil. Thus, resin substitution is requested instead of petroleum-derived resin.
As a substitution of petroleum-derived resin, polylactic acid (hereinafter, it may be abbreviated as “PLA”.) series resin is commonly known. PLA series resin is a plant-derived resin made from lactic acid as a material obtained by fermentation of starch; PLA can be mass-produced in way of chemical engineering and exhibit properties like excellent transparency and stiffness. Therefore, a PLA series resin has gotten a lot of attention in the fields of film packaging materials and of injection molding as a material of substitution having superior shrinkage property in low temperature to that of polyethylene terephthalate series resin, polystyrene series resin, and the like, and having excellent stiffness.
However, as a heat-shrinkable film composed of PLA series resin shows sensible change of heat shrinkage ratio to shrinking temperature, it is difficult to obtain uniform shrinkage. Thereby the film has a problem in shrink finishing quality.
While, a laminated film which is a combination of a PO series resin and a PLA series resin is reported (refer to Patent document 1 and Patent document 2.). However, in the film described in Patent document 1, a PO series resin used in the surface layer has low viscosity average molecular weight, i.e. 1000˜7000, the resin cannot realize physicality such as mechanical strength and heat resistance sufficiently. Therefore, it is inappropriate to use the film for the heat-shrinkable film application. In addition, the film described in Patent document 2 has an outer layer containing 35˜80 mass % of filler, thereby the film after elongation does not have transparency and does lack the mechanical strength. Furthermore, the film described in Patent document 2 has many microporous on the surface, therefore the printability and slidability is not sufficient, and it is difficult for the film to be used as labels.
In Patent document 3, a shrinkable sheet is shown, and the sheet is composed of a layer containing a PO series resin as the main component with a layer containing a PLA series resin as the main component. However, since the sheet shown in Patent document 3 is a sheet having the PO layer as the outer layer, when cylindrical seal bag-making is carried out, sealing is difficult to be conducted. Also, this sheet is a shrinkable sheet formed by inflation method. When it is used as a bottle label in which high-shrinkage in low temperature is required, there is a problem that sufficient shrinkage property in low temperature cannot be obtained.
In Patent documents 4 and 5, laminated films having a PLA series resin and a PO series resin or an ethylene-vinyl acetate copolymer (EVA) are shown. However, these inventions provides EVA layer as a surface layer and a back layer of a film for the purpose of giving heat-seal property. Thereby, the effect which the present invention aims for (namely, shrink finishing quality, transparency, solvent seal property, etc.) is not obtained.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-276144    Patent Document 2: JP-A No. 2002-347184    Patent Document 3: JP-A No. 2002-019053    Patent Document 4: JP-A No. 2000-108202    Patent Document 5: JP-A No. 2004-262029